1. Field of the Invention
The present invention relates to a method of transmitting an overhead message, and more particularly, to a method of transmitting an overhead message in a mobile wireless communication system by syncing the slots of the control channel frame with the slots of the overhead message frame.
2. Discussion of the Related Art
The present invention relates to determining transmission and receiving slots of an overhead message for broadcast/multicast transmission in a Code Division Multiple Access (CDMA) 2000 system and mobile stations. The broadcast/multicast transmission is a transmission scheme by which data can be sent from one source to multiple sources. In broadcast mode, the mobile stations in the service area can receive the broadcast data by being in broadcast receiving mode. In multicast mode, the mobile stations in the service area can receive data by subscribing to a specified multicast service.
The broadcast/multicast service (BCMCS) of CDMA 2000 can be described according to the following three categories. The first category relates to carrying out the BCMCS by using a Forward Broadcast Supplemental Channel (F-BSCH) shared by the mobile stations, which are in idle mode. The F-BSCH is appropriate for the broadcast environment, and at the same time, the F-BSCH is useful in transmitting information to unspecified number of mobile stations. Since the information is transmitted to unspecified number of mobile stations, which are in idle mode, before the traffic channel allocated, portion of transmission power from a base station should be fixedly allocated so that the mobile stations near the cell borders can receive the information as well.
The second category relates to carrying out the BCMCS by using a Forward Broadcast Fundamental Channel (F-BFCH) shared by the mobile stations which are in traffic receiving mode. A method of using the F-BFCH is similar to Push-To-Talk (PTT) method in that the F-BFCH is useful in transmitting the information to a specified number of mobile stations. Here, the traffic channel is first allocated before the information can be transmitted. As such, the transmission power of the F-BFCH is controlled based on the status of other mobile stations sharing the channel and therefore, power can be controlled more efficiently.
The third category relates to carrying out the BCMCS by using a Forward Broadcast Supplemental Channel (F-BSCH) shared by the mobile stations, which are in traffic receiving mode. The F-BSCH is useful in transmitting information to a small number of mobile stations in broadcast receiving mode in a cell. Consequently, transmission power consumption for transmitting broadcast service is low, and less Walsh code is used.
In order for the mobile stations to receive the BCMCS, each mobile station has to receive from a base station information related to broadcast/multicast service, information related to channels used to transmit broadcast/multicast service (e.g., frequency, Walsh code, data rate, size of frame, and public long-code mask), and information related to mobile station registration, to name a few. These types of information are usually transmitted periodically via overhead messages, such as a Broadcast/Multicast Service Parameters Message (BSPM), for example. The mobile station in mode to receive the BCMCS receives an overhead message, such as the BSPM.
More specifically, the BSPM can be transmitted periodically via a Paging Channel (PCH) or a Primary Broadcast Control Channel (P-BCCH). FIG. 1 illustrates a method of transmitting the BSPM via the PCH. In FIG. 1, the PSPM-PERIOD_INDEX is 2. Moreover, the transmission duration is defined by (B+1) slots of the PCH, where B is determined by Equation 1.B=2i×16, 0 ≦i≦7  [Equation 1]
In Equation 1, i is transmitted via BSPM_PERIOD_INDEX included in an Extended System Parameters Message.
As for determining the first slot of each BSPM slot cycle, Equation 2 has to be satisfied.
                                          [                          t              4                        ]                    ⁢          mod          ⁢                                          ⁢                      (                          B              +              1                        )                          =        0                            [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, t represents system time of the PCH frame unit. Furthermore, the reason for dividing the system time into four slots is because the duration of the PCH frame is different from the duration of the PCH slot. That is, since the duration of the PCH frame is 20 ms and the duration of the PCH slot is 80 ms, the system time is divided into four slots in order to express the BSPM slot cycle in PCH slot units. According to Equation 2, as shown in FIG. 1, the BSPM slot cycle can be synced with the PCH slot.
FIG. 2 illustrates a method of transmitting BSPM via the P-BCCH. In FIG. 2, BSPM-PERIOD_INDEX is 2, and the duration of the BCCH slot is 80 ms. Moreover, the duration of the BSPM slot cycle is (B+1) slots of the BCCH, and B is determined by Equation 3.B=2i×16, 0 ≦i≦7  [Equation 3]
In Equation 3, i is transmitted via BSPM_PERIOD_INDEX included in a Multi-Carrier Radio Resource (MC-RR) Parameters Message.
To determine the first slot of each BSPM slot cycle, Equation 4 has to be satisfied.
                                          [                          t              4                        ]                    ⁢          mod          ⁢                                          ⁢                      (                          B              +              1                        )                          =        0                            [                  Equation          ⁢                                          ⁢          4                ]            
In Equation 4, t represents system time of the PCH frame unit. According to FIG. 2, the duration of the BCCH is 40 ms and the duration of the BCCH slot is 80 ms. If the first slot of the BSPM slot cycle is calculated using Equation 4, the BCCH frame slots are not in sync. This is where the problem lies. In other words, since the first slot of the BSPM slot cycle can be selected from any one of 40 ms, 80 ms, and 160 ms based on the BCCH data rates, the first slot of the BSPM slot cycle cannot be synced with the BCCH slot. In short, the slots of the overhead message transmission cycle cannot be synced with the slots of the BCCH.